1. Field of Invention
The invention relates to ceramics. More particularly, this invention pertains to a ceramic material product suitable for use in mirrors, optics, structural ceramics, and the like, and a method for manufacturing the same.
2. Description of the Related Art
Ceramic materials are used in applications such as mirrors, optics, structural ceramics, and the like. In many mounting and structural applications, ceramic materials are desirable for use due to the high achievable purity and complex structural shapes possible with a ceramic material, as well as the relatively high stiffness and low creep of ceramic materials. In other ceramic applications, a lightweight ceramic material is desirable.
A common method of manufacture of structures made of ceramic materials is to sinter components of ceramic materials to form the structure using hot pressing. In a hot pressing process, particles of ceramic material are subjected to elevated temperature and then subjected to increased isostatic gas pressure in an autoclave. An inert gas is used to discourage chemical reaction of the ceramic material. The increased temperature causes the ceramic material to undergo a process called sintering, whereby the particles adhere to each other. Thereafter, the increased pressure and temperature encourages grain boundary diffusion to allow for increased densification of the structure.
In using a hot pressing process to manufacture ceramic materials, pressures exceeding 2,000 psi and temperatures in the range of 1,500 to 2,240 degrees Centigrade, depending on the particular constituent materials used, are necessary to achieve sufficient adhesion of the ceramic particles. The need to achieve and maintain such high temperatures and pressures makes hot-pressing manufacture of ceramics a costly endeavor, thereby resulting in increased cost to the consumer of the ceramic material.
Reaction bonding has been used as an alternative in manufacturing ceramic structures. In reaction bonding, a composite is formed of ceramic particles bonded in a matrix of in situ formed ceramic material. In this process, ceramic particles are mixed with carbon and a sintering aid, such as silicon. The mixture is then heated to a point at which a portion of the carbon and the sintering aid react to form a composite ceramic consisting of ceramic particles distributed throughout a matrix of ceramic material. This reaction results in a semi-continuous phase of sintering aid distributed throughout the composite, with discontinuous ceramic material phases bonding discontinuous phases of ceramic particles.
The reaction bonding process poses several attractive advantages over hot pressing. Less pressure and temperature are necessary to carry on the reaction bonding process as opposed to hot pressing, thereby making reaction bonding more economical. Also, reaction bonding is accomplished using the relatively inexpensive raw materials of the ceramic materials, such as silicon and carbon together with the ceramic particles.
However, despite the advantages of reaction bonding, the performance and quality of reaction-sintered ceramic composite material has traditionally been deemed inferior to hot-pressing manufactured ceramic material. Ceramics inherently contain flaws such as micro cracks, porosity, voids, impurities, and residual stresses from processing that can serve as sites for initiation of failure. U.S. Pat. Nos. 7,104,177; 6,995,103; and 6,862,970 each disclose the use of silicon as an agent to react with carbon and form silicon carbide as a phase that bonds a filler ceramic, either boron carbide or silicon carbide, together with approximately 10-20 percent of unreacted silicon remaining in the composite. Certain publications have theorized that this amount of excess silicon is deleterious to the structural integrity of the finished ceramic material. As is set out in V. Domnich and Y. Gogotsi, Phase Transformation in Silicon Under Contact Loading, Rev. Adv. Mater. Sci. 3, 1-36 (2002), the amount of excess silicon ultimately leads to decreased overall strength and toughness of the finished ceramic material.
Moreover, in traditional reaction bonding, organic materials, such as graphite, are added to the suspended ceramic particles without assuring that the organic materials would cover the surface of all suspended ceramic particle grains. As a result, the reaction bonded ceramic material occurs in a discontinuous phase throughout the composite, with uneven distribution of ceramic material and relatively low surface area contact between the ceramic material and filler particles. This lack of uniformity of ceramic material distribution leads to imperfections within the ceramic composite, which in turn leads to decreased strength and toughness of the ceramic composite. Thus, quality control of the resulting ceramic composite is difficult to maintain using traditional reaction bonding techniques. Furthermore, the use of silicon as a continuous phase is hampered by the tendency for molten silicon to vaporize at temperatures above 1,414 degrees Centigrade. As silicon vapor escapes the colloidal mixture, it leaves behind degassing channels, which weaken the structural integrity of the ceramic product.
In order to apply reaction bonded ceramic composites to a wider range of applications requiring stronger and tougher ceramic materials, more uniform bonding between the suspended ceramic particles and the in situ formed ceramic material is important. Ceramic bodies tend to exhibit stronger and more reliable properties when they are uniformly fine grained, fully dense, and non-porous. In sintered ceramics, a higher glassy grain boundary phase is associated with lower strength, thus indicating that a ceramic with smaller grain size and reduced amount of second “bonding” phase exhibits improved structural properties and resistance to structural failure. Thus, suspending small particles of a relatively hard ceramic material, such as boron carbide, within a uniform and substantially continuous phase of softer yet tougher ceramic material, such as silicon carbide, would allow for a composite ceramic material capable of exhibiting increased overall strength and toughness.
As has been mentioned above, boron carbide and silicon carbide are sometimes used as filler materials for ceramic composites. However, other possible fillers are known. In particular, under certain conditions boron carbide (chemical formula B4C) reacts with silicon to form a crystal lattice in which silicon atoms and boron atoms are substituted for some of the carbon atoms within the lattice. The substitution of larger silicon atoms for carbon atoms at certain positions in the lattice framework results in an expanded lattice with the chemical formula B12(B,C,Si)3. The expansion of the crystal lattice also allows for the insertion of atoms into the lattice framework. Overall, the expansion of the lattice and the insertion of atoms into the lattice framework create a filler material having physical properties more desirable than ordinary boron carbide. See R. Telle, “Structure and Properties of Si-Doped Boron Carbide,” The Physics and Chemistry of Carbides; Nitrides and Borides (Ed. by R. Freer, Kluwer Academic Publishers, Netherlands, 1990), 249-67. Hereafter, the expanded lattice formed from the reaction of silicon with normal boron carbide will be referred to as “expanded lattice boron carbide.”
In manufacturing a reaction bonded ceramic composite having a fine grained filler distribution; a problem arises in appropriating a filler material of sufficiently small grain size. Specifically, finely divided ceramic material suitable for use as filler is significantly more expensive than large grained ceramic material of equal purity. Moreover, a second problem arises in disbursing organic materials evenly throughout the fine grained filler material. In traditional processes of dividing a large grained ceramic material into a more fine grained material, the opportunity for contaminants to adhere to the surface of the filler granules is great. Similarly, in traditional processes of dividing carbon and other organic materials into material of sufficient grain size to allow for disbursement within the fine grained filler material, the opportunity for contaminants to oxidize portions of the organic material is great. Such contamination of the fine grained filler material and the organic material leads to reduced bonding between the filler material and the organic material, thereby reducing the amount of surface to surface contact between the continuous-phase in-situ-formed ceramic material and the filler material. Such reduced surface-to-surface contact ultimately results in greater instances of weak points in the product ceramic composite.